Cracking process



June 28, 1938 l ,A. c. PATTERSON I. 2,121,978

V CRACKING PRocEss Filed July 11, 193s CON-DEIYS 24-11 g.; *f

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Patented `lune v28, 1938 UNITED STATES PATENT A' GRACKING PROCESS Albert O. Patterson,` RoselleffN. J., assigner to y Standard Oil Development Company, a corporation of Delaware Application July 11, 1936, Serial No. 90,045

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AIn 'all processesin which a virgin stock, such asta 'reduced crude or a virgin gas oil, is cracked, the'amount of crackable material convertible into Alower boiling products is limited'by coke production; That is tol say, the cracking conditions vcan be only so intense that the amount of coke formed does not plug up the system. In practice, excessive coke formation occurs before all the crackable material in the stock is converted. In the past,

this has resulted in a loss of cracking stock in l thetar resulting from the cracking.

' vFor many years, in the operation of two circuit systems, it has'been the practice to discharge the cracked virgin stock and thecracked cycle stock into a common separator. In'manyinstances the cracked effluent from both cycles has been joined at a point Vjust before their entry into the separator. More often the cracked cycle stock has been introduced into the separator at a point above the point of introduction of the cracked virgin stock, probably by reason of the fact lthat the cycle stock is usually of a lighter character. l Recently it has been proposed to introduce the cracked cycle stock into the separator at a point below the point of introductionof the cracked less refractory stock and to interpose fractionating equipment between the two pointsofintro-V n duction. This has resulted in a preferential strip-v ping of good cracking stockfrom the cracked effluent from the less refractory stock by the cracked cycle stock andthe replacement of this good cracking stock in the tar by refractory constituents of substantially the same boiling point from the cracked effluent from the cycle stock.

:In the two-circuit systems, to ywhich the recently proposed improvement has been directed, the cracking coil for the less refractory stock is ordinarily operated at temperatures rangingfrom 750 to 1000 F.,vusually from 820 to 900 F., and

5U the coil'for the cycle stock is operated at an outlet temperature between 850 and 1050 F., usually between 900 and. 975 F. 'Ihe pressure on the less-refractory stock coil is ordinarily between 100 and 500 lbs/sq. in., and that on the cycle stock 155- coil is `between 300 and1000 lbs/sq.v in., and that on the separator is much'lower, usually between and 150 lbs/sq. in. In such operations',-'theamount of good cracking stock recovered from the `cracked less refractory material, while it is considerably larger than. the amountobtainable. without'the employment of the expedient of `fractionation in the separator accordingto the .recent proposal, is still limited by the formation of coke under the operating conditions. in the'separaton The lower the pressure in thev separator, the greater will be ther amount of coke'formed at a given temperature.

According to the present invention, va greater recover'ylof cracking stock from the tar can be achieved by separately collecting from the separating Zone the tar'from the less refractory stock and thev tar from the more refractory stock, and intrcducingthese' tarsinto avcommon evaporator operated at a lower pressure, preferably a vacuum' flash tower, at different levels, the tar from the less refractory stock being introduced at a point above the point o-f introduction of the tar from the more refractory stock, and fractionating equipment 'being interposed between them. By reason of the reduced pressure in this common evaporator it is possible to operateat lower temperatures whereby the respective tarscan be reduced to a lower A.- P. I. gravity number without the danger of coking. Thisnov'el procedure can be'applied regardless of the ccnditionsof opera-fy tion in the respective circuits, andis not `restricted to circuits operated under the general conditions outlined above, which are given merely as exemplary of conventional operation. In some instances it may be preferably to operate the cycle stock circuit at a'lower pressure than that maintainedA in the less refractory stock circuit. Itl is generally true,fhowever, that the temperature in the cycle stock circuit will be higher than cedure results in a considerable loss "of '-'he'at.'

Likewise, the amount of heat `contained-in the overhead is limited bythe pressurein vthesepa'- rator. This reduces the amountv of 'preheat which can be-impartedv to the charging stock which is ordinarily passed in heatV exchange'l relation with the overhead from the separatori'.

'According to a preferred embodiment'of the present invention, the advantages' of increased preheat to the charging stock and avoidance of loss of heat by cooling the tar in the bottom of the separator, together with the advantage accruing from stripping the virgin stock by means of the cycle stock, and the additional advantage of the avoidance of coke formation, are obtained by injecting both the cycle stock and the virgin stock either into individual separators maintained at the pressures held in the respective cracking coils or into a common separator maintained at a high pressure and so constructed as to provide for stripping of the virgin stock by vapors of" the cracked cycle stock, while keeping the tars resulting from the respective stocks separate, followed by injection of the tars from the respective stocks into a common evaporator operating at lower pressure, preferably, a vacuum flash tower, the tars from the cycle stock being introduced at a point below the point of introduction of a tar from the virgin stock, and fractionating equipment being interposed between the points of introduction.

Further procedural details and advantages of the process of the present invention will be apparent from the following detailed description of the accompanying drawing which is a front elevation, partly in section, of a diagrammatical form of an apparatus suitable for carrying out the preferred embodiment of said process.

Referring to the drawing in detail, virgin stock, such as reduced crude or virgin gas oil, is introduced into the system through pipe I from which it is pumped by pump 2, through line 3, through heat exchanger 4, referred to in greater detail hereinafter, to coil 5 from which it is discharged into a soaker 6, which is shown as downow but may be upflow, and thence into the upper part of separator 1 provided with fractionating plates 8, which may be replaced by other common fractionating equipment, and with a tray 9, the purpose of which is to collect the tar from the cracked virgin stock and feed it through line IIJ into vacuum flash tower II.

The overhead from separator 'I passes through line I2 into initial heat exchanger 4, which contains the heating coil I4 for the virgin feed stock, and then to a second heat exchanger I3, and then into bubble tower I5, from which gasoline and lighter hydrocarbons are taken oif overhead through line I6 and cycle gas oil is withdrawn from the bottom through line I1.

The initial condensate from the overhead from separator 'I is withdrawn from heat exchanger 4, through line I8 and reintroduced into the top of separator to act as reflux. The condensate in heat exchanger I3 is withdrawn through, line I9 and added to the cycle gas oil in line II, the mixture being passed throughk coil 20 in heat exchanger I 3 and thence to cracking coil 2|, from which the cracking products are discharged into the bottom of a soaker'22, which may be downflow as well as upllow, from which they pass to the bottom of separator I at a point below tray 9.

The vapors from the cracked cycle stock pass upwardly through the central opening 23 in tray 9 and serve to strip the downwardly iiowing liquid products from the cracked virgin stock. The tar resulting from the cracking of the cycle stock is withdrawn from separator 'I, through line 24, and charged into vacuum flash tower II at a point below the point of introduction of line I. Between the points of introduction of the two tars are fractionating plates 25. The final tar of a gravity much below that which can be obtained without excessive coking in separator 1 is withdrawn through line 26, and the overhead, which is composed mainly of hydrocarbons bolling within the gas oil range, is removed from tower II through line 21 and passed through a condenser 28 into an accumulator 29, from which it is pumped through preheater 30 and added to the feed to coil 2I.

Aline 3Iv connects line I9 with line I8 and valves 32, 33 and 34 are provided so that by suitable manipulation the condensates from either or Iboth of exchangers 4 and I3 may be sent to the top of the separator as reflux, or to line II asrecycle stock.

A pressure reducing valve 35 is provided in line I2 at a point just before bubble tower I5 and in lines I and 24 at a point just before flash tower II. Since the cycle stock cracking coil is often operatedk at a higher pressure than the virgin stock cracking coil, it is advisable to have a pressure reducing valve 36 in the line between soaker 22 and separator l, so that the pressure maintained in separator l can be substantially the same as that maintained in the virgin stock cracking circuit. In the event that each cracking circuit is provided with an individual separator, there need be no reducing pressure valve between the cycle stock cracking equipment and its-corresponding separator. A greater amount of preheat can be obtained from the overhead from this individualseparator than can be obtained from the cracked cycle stock in the arrangement shown in the drawing. The additional preheat, however, probably does not compensate for the extra equipment that would be required in such an arrangement, althoughthis arrangement is contemplated by the present invention. To provide for operation of the separator at a lower pressure than that maintained in either cracking circuit, a reducing valve 31 is provided between soaker Ii and the separator.

The conditions of operation in a cracking system of the type described have been previously stated. ln the preferred embodiment of the process of the present invention, as distinguished from the previous operation, the pressure in the separator where a common separator is employed will be substantially the same as that in the lower pressure cracking, zone. Where individual separators Yare employed, the pressure in separator 'I will be substantially the same as the pressure in its corresponding cracking zone, the difference in pressure between the two in any case being only the drop V,in pressure resulting from the passage of the reaction material through the equipment. In either case, pressure is maintained on the overhead products from the separator to a point just before the introduction of these products. into the bubble tower, which is normally operated-.at a considerably lower pressure, in some cases as low as atmospheric pressure. The pressure on the vacuum flash Zone is ordinarily less than atmospheric, preferably about 50 mm. of mercury.

In a specic practical operation in the apparatus shown in the drawing, the temperature at the outlet of coil 5 is 840 F. `and at the outlet of coil 2-is 877F. The cracked effluent enters the separator at a temperature of about 800 F. The pressure on coil vandsoaker 6 is about 300 lbs/sq. in. andon coil-2l and soaker 22 about 750 lbs/sq. in.,

lThe separator `Il is held under a pressure of about 90lbs./sq. in. The temperature at the bottomof the separator is about'l'O" F. and at the outlet of tray 9 about 720 F. The temperature at the top of the separator is about 680 F. The pressure in the bubble tower is about 80 lbs/sq. in. and in ash tower Il about 50 mms. of mercury.

The nature and objects of the present invention having been thus described and the preferred embodiment of the same having been illustrated, what is claimed as new and useful and desired to be secured by Letters Patent is:

1. In a two-circuit cracking process, in which two stocks of different degrees of refractoriness are separately cracked under pressure, the steps whichy comprise discharging saidl cracked stocks into separate separating zones maintained under superatmospheric pressure, separately collecting a tar from eaclrof said separating zones .and discharging said tars'into a common vaporvizing zonemaintained under a lower pressure, `the tar from the more refractory .stock being introduced into said zone at a point below the point of introduction of the tar from the less refractory stock and the liquid constituents of the latter being impeded in their downward flow and being passed counter-currently to the vapors evolved from the former.

2`. In a two-circuit cracking process, the steps rwhich comprise subjecting a heavy stock to cracking conditions under superatmospheric pressure, subjecting a more refractory stock to more severe cracking conditions, discharging cracking products from both stocks into acommon separating zone maintained under superatmospheric pressure,'impeding the downward flow of the liquid constituents of the cracked less refractory stock, passing the vapors from the cracked more refractory stock upwardly in counter-current to said liquid constituents, separately collecting a tar from the cracked less refractory stock and the cracked more refractory stock, discharging said tars at different levels into a common vaporizing zone maintained under reduced pressure, the tar from the less refractory stock being lintroduced into said zone at a point above the point of introduction of the tar fromy said more refractory stock, impeding the downward flow of the liquid constituents of the tar from the said less refractory stock and passing vapors from the tar from said more refractory stock upwardly in counter-current to said liquid constituents, withdrawing a combined tar from the bottom oi said zone and withdrawing a lower boiling material from the vtop of said Zone.

3. A process according to claim l in which the separate stocks are cracked under superatmospheric pressure and discharged into individual separating zones maintained under substantially the same pressure as its corresponding cracking 

